This study examines the effects of professional development on various aspects of teachers' mathematics instruction. Using data collected between 2005 and 2009, we examined the extent to which the instructional practices of 49 US high school teachers who participated in content-based, sustained professional development changed over time. We found that changes in several aspects of their instructional practices followed somewhat different patterns. Teachers made statistically significant and steady changes in mathematical discourse, instructional clarity, and the development of students' mathematical habit of mind, but not in student interactions or in the use of multiple representations.